High strength reduced be casting alloy

ABSTRACT

The Be content of Be Al alloys suitable for investment casting, which contain a small but suitable amount of Ag, can be significantly reduced without adversely affecting their thermal or investment casting properties by including significantly more Si in the alloy than done in the past.

BACKGROUND

This invention relates to improved Be Al alloys for use in investmentcasting.

As described in commonly-assigned U.S. Pat. No. 5,667,600 to Grensing etal., the disclosure of which is incorporated herein by reference,investment casting is a type of casting normally used to make metalparts of complex shape. “Investment casting” connotes that the castingis produced by using a mold that is made from a ceramic shell. The useof a ceramic shell increases the maximum allowable casting temperaturethereby permitting the casting of relatively high-melting-temperaturealloys. In general such alloys cannot be cast using other proceduressuch as injection casting or sand casting. Casting produces componentsthat are “near net shape,” i.e., a shape which is very near to the shapeof the final product to be made. Consequently casting, and investmentcasting in particular, is desirable because it essentially eliminatesthe extensive machining that would otherwise be necessary to transform ablock of metal into its final desired shape. This reduces both themachining costs and also the amount of metal needed to produce the part.

Be and Al have widely different melting temperatures, 1289° C. and 660°C. respectively. This makes investment casting of Be Al alloys verydifficult, because this large difference in melting temperatures leadsto large differences between the liquidus and solidus temperatures ofthese alloys. See, U.S. Pat. No. 5,603,780 to Nachtrab et al., thedisclosure of which is also incorporated herein by reference, especiallycol. 1, lines 31 to 50. This, in turn, often leads to excessiveporosities, coarse microstructures or both in castings made from thesealloys. Accordingly, shaped parts made from Be Al alloys are mostcommonly made by powder metallurgy techniques.

The above-mentioned Grensing et al. patent indicates that Be Al alloyssuitable for investment casting can contain as little as 30 wt.% Be.However, experience has shown that Be contents of at least about 56 wt.% and more commonly about 61 to 69 wt. % are necessary to makecommercially-acceptable alloys, i.e., alloys exhibiting acceptablelevels of segregation and microporosity. For example, the three Be Alinvestment casting alloys available on the market today have Be contentsof 56 to 68 wt. %. Note, also, that the alloys in all working examplesof the Grensing et al. patent have Be contents of at least 62 wt. %.Also note the express disclosure in the Nachtrab et al. patent mentionedabove that Be must be present in an amount of at least about 60 wt. %.

Two desirable properties of Be Al investment casting alloys are lowcoefficients of thermal expansion and strength. In this regard, allthree commercial Be Al investment casting alloys mentioned above havecoefficients of thermal expansion of about 14.5 μm/m (ppm) or less. Inaddition, two of these alloys have 0.2% yield strengths of about 130 to155 MPa and ultimate tensile strengths of about 200 MPa, while the thirdhas a 2% yield strength of about 200 MPa and ultimate tensile strengthsof about 255 MPa.

Beryllium is expensive. Therefore, it would be advantageous if theconcentration of Be in these alloys could be reduced without adverselyaffecting their strength, thermal and investment casting properties. Andit would be especially desirable if this could be done in such a waythat one or more other properties of these alloys were actuallyimproved.

SUMMARY

In accordance with this invention it has been found that, by adding Ni,Co and/or Cu to certain Ag-containing Be Al investment casting alloys,and in addition by increasing the Si content of these alloys beyondconventional amounts, it is possible to reduce the Be content of thesealloys while simultaneously increasing their strength without adverselyaffecting other important properties of these alloys such as theircoefficients of thermal expansion and their ability to be investmentcast with minimal segregation and microporosity.

Thus, this invention provides a new Be Al alloy suitable for investmentcasting purposes, the alloy comprising about 40 to 55 wt. % Be, about1.3 to 7.0 wt. % Ag, about 1.5-10 wt. % Si, about 0.5-6.0 wt. % of Ni,Co, Cu or mixtures thereof, and no more than about 3 wt. % optionalingredients, with the balance being Al plus incidental impurities,wherein the Be/Al ratio of the alloy is <˜2.0, the combined amounts ofSi and Ag in the alloy is >˜4 wt. %, the combined amounts of Si, Ag, Ni,Co and Cu in the alloy is ≧˜6 wt. %, and further wherein the alloy has acoefficient of thermal expansion of ≦˜15 μm/m (ppm), a 0.2% yieldstrength of ≧˜200 MPa and an ultimate tensile strength of ≧˜250 MPa.

DETAILED DESCRIPTION

The inventive Be Al investment casting alloys contain significantly lessBe than known Be Al alloys exhibiting commercially desirable investmentcasting properties. Thus, the inventive Be Al alloys normally containabout 55 wt. % or less Be, more commonly about 50 wt. % or less.Normally, they also contain at least about 40 wt. % Be, because themodulus of elasticity and specific stiffness of Be Al alloys containingless Be are simply too low for many investment casting applications.Thus, the inventive Be Al investment casting alloys desirably exhibit amodulus of elasticity of at least about 140 GPa @ 25° C., preferably atleast about 150 GPa @ 25° C. Alloys containing at least about 45 wt. %Be, and especially about 47 to <˜50 wt. %, Be are particularlyinteresting, as are alloys exhibiting a modulus of elasticity of atleast about 160 GPa @ 25° C. Alloys containing <50 wt. % Be aredesirable, because an export license from the U.S. federal government isnot required.

In addition to Be, the inventive alloys also contain Ag. As indicatedabove, as of this writing, there are three commercially-available Be Alinvestment casting alloys in the United States. Two contain about1.65-3.35 wt. % Ag, while the third contains no Ag. In addition, theabove-noted Nachtrab et al. patent indicates that Be Al investmentcasting alloys can contain up to 4.25 wt. % Ag. The inventive Be Alinvestment casting alloys contain similar amounts of Ag as theseAg-containing alloys. Thus, the inventive Be Al alloys contain at leastabout 1.3 wt. % Ag, and more commonly at least about 1.5 wt. % or evenat least about 2.0 wt. % Ag. In addition, they may contain as much asabout 7.0 wt. % Ag, but more commonly will contain no more than about6.0 wt. %, no more than about 5.0 wt. % or even no more than about 4.0wt. % Ag. Ag contents of about 1.3-7.0 wt. %, about 1.5-6.0 wt. %, oreven about 2-5 wt. %, are more interesting.

In addition to Be and Ag, the inventive Be Al alloys also contain Si.Two of the commercial investment casting Be Al alloys mentioned abovecontain no Si while the third contains about 1.65-2.5 wt. % Si.Similarly, the above-noted Nachtrab et al. patent, although indicatingthat the Si can be present in its alloys in amounts as high as 4 wt. %,shows in its working examples that Si content as a practical matter islimited to a maximum of 2.0 wt. %. The inventive alloys differ fromthese alloys in that they contain significantly more Si for enhancedcastability. Thus, the inventive alloys contain at least about 1.5 wt. %Si, with alloys containing at least about 2.0 wt. % Si or even at leastabout 2.5 wt. % Si being more interesting.

As for maximum Si content, the Si concentration preferably does notexceed the value needed to keep essentially all of the Si in the Al—Sieutectic phase that is believed to be formed in the inventive alloys. Asfurther discussed below, the inventive alloys are believed to differfrom conventional Be Si alloys in that the inventive alloys generallycontain more Si. Consequently they form more Al—Si eutectic phase, butthey also still contain primary Al cells, i.e., cells of essentiallypure Al. Accordingly, the maximum Si content in these alloys ispreferably selected so that essentially all of the Si in these alloysremains in this eutectic phase. Generally speaking, this means that themaximum Si content of the inventive alloys will about 7 wt. %, althoughmaximums on the order of about 6 wt. %, about 5 wt. %, about 4 wt. % oreven about 3 wt. % are more common.

The inventive Be Al alloys also contain a suitable amount of Ni, Co, Cuor mixtures thereof. Alloys containing mixtures of Ni and Co areespecially interesting. In accordance with this invention, Ni, Co and/orCu are included in the inventive alloys to increase strength. Inaccordance with this invention, it has been found that the presence ofone or more of these elements will substantially increase both 0.2%yield strength as well as ultimate tensile strength, provided that thealloys also contain at least about 1.3 wt. % Ag as well as at leastabout 1.5 wt. % Si. Such alloys containing at least about 2.0 wt. % Sior even at least about 2.5 wt. % Si are more interesting.

The amount of Ni, Co and/or Cu that should be included in the inventivealloys will normally be at least about 0.5 wt. %, but more commonly atleast about 1 wt. % or even at least about 2 wt. %. The maximum amountof Ni, Co and/or Cu in the inventive alloys will normally be no morethan about 6 wt. %, but more commonly no more than about 5 wt. %, oreven no more than about 4 wt. % so as not to adversely affect thermalconductivity or castability in a significant way. Therefore, theinventive alloys will typically contain about 0.5 to 6 wt. %, morecommonly about 1 to 5 wt. % or even about 2 to 4 wt. % of Ni, Co, Cu ormixtures thereof.

The inventive Be Al alloys can also contain elements known to increaseductility such as Sr, Na, Ca and Sb. If so, the amount of suchingredients in these alloys should be no greater than about 0.3 wt. %,more desirably no more than about 0.25 wt. %. Alloys containing about0.005 to 0.2 wt %, about 0.01 to 0.1 wt. %, or even about 0.02 to 0.08wt. % of these elements are more interesting. Alloys containing about0.02 to 0.06 wt. % or even about 0.03 to 0.05 wt. % Sr are especiallyinteresting.

Other known ingredients in Be Al alloys can also be included in theinventive Be Al alloys. Examples include Ge, Ti, Zr, B, Sc, Y and therare earth elements. If so, the total amount of such ingredients shouldnot exceed about 1.0 wt. %, preferably about 0.5 wt. %., about 0.3 wt. %or even about 0.1 wt. %. Alloys which are essentially free of theseingredients are preferred.

The inventive Be Al alloys can also contain other optional ingredientswhich do not adversely affect the properties of the alloys in anysignificant way. Normally, the total amount of these optionalingredients will not exceed about 3.0 wt. %, preferably not exceed about2.0 wt. %, about 1.0 wt. % or even about 0.5 wt. %.

The balance of the inventive alloys is Al and incidental impurities. Byincidental impurities is meant ingredients which are present in suchsmall amounts (usually trace amounts) that their effect on theproperties of the alloy obtained are insignificant. As well appreciatedin metallurgy, it makes no economic sense to refine out tracecontaminants which have an insignificant effect on alloy performance.The same considerations apply here.

As indicated above, it has been found in accordance with this inventionthat the strength of certain Ag-containing Be Al investment castingalloys can be significantly increased without adversely affecting theirinvestment casting properties or their coefficients of thermal expansionby including Ni, Co and/or Cu in the alloy and, in addition, bydecreasing their Be contents and increasing their Si contents relativeto similar alloys known in the past. This compositional modification isreflected by at least three different features of the inventive alloyscompared with conventional alloys.

First, the inventive alloys contain substantially less Be andcorrespondingly more Al, on a relative basis, than conventional alloys.This is reflected by the fact that, in the inventive alloys, the Be/Alratio is <˜2.0, more desirably ≦˜1.6 or even ≦˜1.4. In contrast, in theSi-containing commercial alloys mentioned above as well as all of theSi-containing alloys specifically disclosed in the above-mentionedGrensing et al. and Nachtrab et al. patents, the Be/Al ratio is ≧2.0.The difference between the inventive and earlier alloys in terms of Alcontent is also reflected by the fact that the Al content of theinventive alloys is typically ≧˜34 wt. %, more commonly ≧˜37 wt. % andeven ≧˜40 wt. %. In the Si-containing commercial alloys mentioned aboveas well as all of the Si-containing alloys specifically disclosed in theabove-mentioned Grensing et al. and Nachtrab et al. patents, the maximumAl content is 33 wt. %

The second difference between the inventive alloys and conventionalalloys is that the combined amount of Si and Ag in the inventive alloysis greater than in conventional alloys. This is reflected by the factthat, in the inventive alloys, the combined amount of Si and Ag is >4.0wt. %, more desirably ≧˜5.0 wt. %, or even ≧˜5.8 wt. %. Although thegeneral disclosure of the above-noted Nachtrab et al. patent indicatesits alloys can contain up to 4 wt. % Si and up to 4.25 wt. % Ag, theworking examples of this patent show that the combined amount of theseelements is limited to a maximum of 4.0 wt. %. Meanwhile, the maximumcombined amount of Si and Ag in the above-noted commercial alloys whichalso contain Ni, Co or Cu, as well as the specific alloys described inthe above-noted Grensing et al. patent which also contain Ni, Co or Cu,is 3.35 wt. %.

The third difference between the inventive alloys and conventionalalloys is that the combined amounts of Ag, Si, Ni, Co and Cu in theinventive alloys is ≧˜6.0 wt. %, more typically ≧˜7.0 wt. %, or even≧˜8.0 wt. %. In contrast, the combined amounts of Ag, Si, Ni, Co and Cuin the specific alloys described in the above-noted Nachtrab et al. andGrensing et al. patents which contain at least one of Si and Ag, as wellas at least one of Ni, Co and Cu, is 4.25 to 5.9 wt. %. Similarly, thecombined amounts of Ag, Si, Ni, Co and Cu in the specific commercialalloys mentioned above which contains at least one of Si and Ag, as wellas at least one of Ni, Co and Cu, is 3.30 to 4.70 wt. %.

From the above, it can be seen that the inventive alloys differ fromconventional Be Al investment casting alloys in that the inventivealloys not only contain less Be and more Al than their conventionalSi-containing counterparts but also more Si plus Ag, as well as agreater total amount of Si, Ag and Ni, Co and/or Cu. It is well knownthat the microstructure of a Be Al investment casting alloy is composedof a Be phase (Be-based dendrites) surrounded by an Al matrix. Althoughnot wishing to be bound to any theory, it is believed that the increasedamounts of Al and Si in the inventive alloys generate a modifiedmicrocrystalline structure in which the Be phase is surround by an Al—Sieutectic phase which also contains primary Al cells, i.e., cells ofessentially pure Al. This Al—Si eutectic, it is believed, is responsiblenot only for the low microporosity exhibited by the inventive Be Alalloys but also their desirable coefficients of thermal expansion. Inaddition, it is further believed that the substantially improvedstrengths exhibited by these alloys is due to the incorporation of theNi, Co and/or Cu in these alloys into this Al—Si eutectic.

In this regard, the inventive Be Al alloys exhibit a desirably lowcoefficient of thermal expansion of ≦15.0 μm/m (ppm), more desirably≦14.8 μm/m (ppm), ≦14.6 μm/m (ppm) or even ≦14.5 μm/m (ppm). Inaddition, they also exhibit a superior 0.2% yield strength of ≧200 MPa,more desirably ≧220 MPa, and even ≧230 MPa, as well as a superiorultimate tensile strength of ≧250 MPa, more desirably ≧265 MPa, and even≧280 MPa. This low coefficient of thermal expansion is essentially asgood as that exhibited by the above mentioned commercial alloys, whilethis superior 0.2% yield strength and ultimate tensile strength arebetter than that exhibited by such alloys. That is to say, preferred BeAl alloys of this invention exhibit 0.2% yield strengths of 220 MPa orhigher and ultimate tensile strengths of 265 MPa or higher, which isgreater than that of other known Be Al investment casting alloys. Inaddition, the inventive alloys also generally exhibit a modulus ofelasticity of less than 167 GPa @ 25° C. and a density greater than 2.2gm/cm³.

WORKING EXAMPLES

In order to more thoroughly describe this invention, the followingworking examples are provided. In each of these working examples, Be Alalloys were made by the general procedure described in the above-notedGrensing et al. patent in which castings of each alloy were made bycharging a superheated molten mass of the alloy into a heated mold undersuitable vacuum conditions. After cooling and removal from the mold,each alloy was subjected to a series of standard analytical tests todetermine its properties.

Five different alloys were tested. Three of these alloys represent theconventional commercially-available alloys mentioned above, these alloysbeing to referred to as Commercial Alloy A (“CA-A”), Commercial Alloy B(“CA-B”), and Commercial Alloy C (“CA-C”). A fourth recently developedalloy which is the subject of another patent document, which is referredto as Developmental Alloy-D (“DA-D”), was also tested, as was a fifthalloy representing this invention.

The compositions of these alloys as well as the results obtained are setforth in the attached Table 1. For the purposes of comparison, thechemical composition of the alloys shown in Examples V-VII of theabove-noted Natchrab et al. patent as well as the chemical compositionof the alloy shown in Example 5 of the above-noted Grensing et al.patent are also included in Table 1.

TABLE 1 Composition and Properties of Alloys in Working Examples NR GrUnit Ex 5-7 Ex 5 CA-A CA-B CA-C DA-D Invent Composition Be wt. % 65 6461.1-68.6 56-63 61.1-68.6 49.5 49.5 Al wt. % 31 30 bal (~31) bal (~47)bal (~30) 44 41.5 Si wt. % 2 1.4 1.65-2.50 4.5 3.0 Ag wt. % 2 1.51.65-2.35 2.65-3.35 2.0 3.0 Ni/Co/Cu wt. % 0.25 3 Ni 2.4-3.2 0.65-1.353.0 other wt. % 0.04 0.1 0.55-0.95 0.04 0.04 Sr Ti Ge Sr Sr Be/Al 2.1~2.1 ~2.1 ~1.28 ~2.2 1.125 1.20 Si/Al 0.065 0.047 0.05 0.102 0.072 Ag +Si wt. % 4 2.9 3.30-4.85 2.65-3.35 6.5 6.0 Ag/Si/Ni/Co/Cu wt. % 4.25 5.93.30-4.85 2.4-3.2 3.30-4.70 6.5 9.0 Properties Density g/cm³ 2.16 2.162.16 2.21 2.21 Melt (Liquidus) ° C. 1287 1287 1287 1287 1287 CTE μm/m13.4 14.6 14.2 13.6 14.4 Thermal W/M- 180 110 105.5 186 98.1Conductivity ° K. @ 30° C. Modulus of GPa 202 202 202 167 167 Elasticityin @ Tension 25° C. Specific 93.5 93.5 93.5 75.5 75.5 Stiffness 0.2% YSMPa 137.9 151.7 200 145 235 UTS MPa 196.5 200 255 200 290 % Elongation %@ 1.7 5.4 3.4 3.0 2.0 25° C.

From Table 1, it can be seen that the amount of Be in the inventive BeAl alloy is about 23% less than the commercial alloys containing Si(including the Si-containing alloys of the Natchab et al. and Grensinget al. patents) and over 17% less than the commercial alloy containingno Si or Ag. Since Be is expensive, this means that the inventive alloyis significantly less expensive to make than these commercial alloys.

From Table 1, it can also be seen that the inventive alloy hasmechanical properties such as % Elongation and coefficient of thermalexpansion comparable to that of the commercial alloys. In terms of 0.2%Yield Strength and Ultimate Tensile Strength, however, the inventivealloy is superior in that its 0.2% Yield Strength and Ultimate TensileStrength are substantially better than that of Commercial Alloys A, Band C as well as Developmental Alloy D.

These data show that, by formulating the inventive alloys as describedherein, not only is it possible to substantially reduce the Be contentof the alloys obtained, but in addition it is also possible to do so ina way which preserves the investment casting properties of the alloyswhile substantially improving their strength at the same time.

Although only a few embodiments of this invention have been describedabove, it should be appreciated that many modifications can be madewithout departing from the spirit and scope of this invention. All suchmodifications are intended to be included within the scope of thisinvention, which is to be limited only by the following claims.

1. A Be Al alloy suitable for investment casting purposes, the alloycomprising about 40 to 55 wt. % Be, about 1.3 to 7.0 wt. % Ag, about1.5-7 wt. % Si, about 0.5-6.0 wt. % of Ni, Co, Cu or mixtures thereof,and no more than about 3 wt. % optional ingredients, with the balancebeing Al plus incidental impurities, wherein the Be/Al ratio of thealloy is <˜2.0, the combined amounts of Si and Ag in the alloy is >4 wt.%, the combined amounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜6 wt.%, and further wherein the alloy has a coefficient of thermal expansionof ≦15 μm/m (ppm), a 0.2% yield strength of ≧200 MPa and an ultimatetensile strength of ≧250 MPa.
 2. The alloy of claim 1, wherein the Be/Alratio is ≦˜1.6.
 3. The alloy of claim 2, wherein the Be/Al ratio is≦˜1.4.
 4. The alloy of claim 1, wherein the combined amounts of Ag andSi in the alloy is ≧˜5.0 wt. %
 5. The alloy of claim 4, wherein thecombined amounts of Ag and Si in the alloy is ≧˜5.8 wt. %.
 6. The alloyof claim 1, wherein the combined amounts of Si, Ag, Ni, Co and Cu in thealloy is ≧˜7.0 wt. %.
 7. The alloy of claim 6, wherein the combinedamounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜8.0 wt. %.
 8. Thealloy of claim 1, wherein the alloy has a 0.2% yield strength of ≧220MPa and an ultimate tensile strength of ≧265 MPa.
 9. The alloy of claim8, wherein the alloy has a 0.2% yield strength of ≧230 MPa and anultimate tensile strength of ≧280 MPa.
 10. The alloy of claim 1, whereinthe alloy contains about 45 to 55 wt. % Be, about 2.0-7.0 wt. % Si,about 1.5 to 6.0 wt. % Ag, and no more than about 1.5 wt. % optionalingredients, about 1-5 wt. % of Ni, Co, Cu or mixtures thereof, with thebalance being Al plus incidental impurities, wherein the Be/Al ratio is≦˜1.6., the combined amounts of Si and Ag is ≧˜5.0 wt. %, the combinedamounts of Si, Ag, Ni, Co and Cu in the alloy is ≧˜7 wt. %, and furtherwherein the alloy has a 0.2% yield strength of ≧220 MPa and an ultimatetensile strength of ≧265 MPa.
 11. The alloy of claim 10, wherein thealloy contains ˜47.5 to <˜50 wt. % Be, about 2.5-6.0 wt. % Si, about 2-5wt. % Ag, about 2-4 wt. % of Ni, Co, Cu or mixtures thereof, and no morethan about 0.5 wt. % optional ingredients, with the balance being Alplus incidental impurities, wherein the combined amounts of Si and Ag is≧˜5.8 wt. %, the combined amounts of Si, Ag, Ni, Co and Cu in the alloyis ≧˜8 wt. %, and further wherein the alloy has a 0.2% yield strength of≧230 MPa and an ultimate tensile strength of ≧280 MPa.
 12. The alloy ofclaim 1, wherein the alloy contains about 0.01 to 0.1 wt. % strontium,sodium, calcium, antimony or mixtures thereof.
 13. The alloy of claim 1,wherein the alloy contains about 0.02 to 0.06 wt. % strontium.
 14. Thealloy of claim 1, wherein the alloy contains less than about 50 wt. %Be.